Reconstruction of the Old Bridge of Mostar M

Acta Polytechnica Vol. 46 No. 2/2006 Czech Technical University in Prague Reconstruction of the Old Bridge of Mostar M. Popovac

The Old Bridge of Mostar was built in 1566 by Hajrudin, a student of Kodza Mimar Sinan, the greatest Ottoman architect. It is a stone bridge of very slender and elegant shapes: its profile and its skyline are so thin and so high over the river waters that it is hard to believe that such a structure could be made out of huge stone blocks. The Bridge was destroyed in November 1993 by shelling during the recent war events. Its reconstruction was one of the biggest and most complicated projects involving UNESCO, The World Bank and many local and international experts. The task was to build a New Old Bridge – precisely the same in all details as the Old one. After many studies, tests and shape determinations, the project was completed and the actual reconstruction work could begin. Ancient techniques and methods, original materials and a perfectly reconstructed shape gave this Bridge its new life in post-war Mostar.

Keywords: old, bridge, Mostar, Maja, Popovac, Bosnia, Herzegovina, reconstruction, UNESCO, World Bank.

Fig. 1: Bridge in the 1980s

1 History became the meeting point of roads from the sea connecting the south to the north and the eastern regions to the western Stari Most (or Old Bridge) is the most recognizable sym- regions. bol of Mostar. Sadly, most people only know Mostar from the The first medieval bridge in Mostar was a wooden suspen- destruction of Stari Most in 1993 during the War in the sion bridge which was very unstable and of fragile construc- Balkans. tion. In the 16th century, Sultan Sulejman the Magnificent The oldest written testament to the existence of medieval ordered the construction of a new bridge in Mostar. From Mostar dates back to the 15th century, prior to the invasion of the reports of Hadzi Kalfa, the bridge was completed in 1566 the Ottoman Turks. The first written documents about the which, coincidentally, was the last year of Sulejman’s reign. city date from 1452. In report to the city council of Dubrovnik There are no documents about the building procedures, about the rebellion of the ruler’s son, who took several towns the site organization or the construction. We think that the into his possession, including the town with “Duo Castelli al centering (semi-circular scaffolding) was made of wood, and pontedeNeretua”. that the bridge was built in a short period of time – maybe Soon after its foundation, Mostar have taken over the pri- even in a single dry season. There is also a tale that Hajrudin mate of Blagaj, an old city situated under the Castel of the escaped to the mountains before the centering was released, Hercegovina region founder – Herceg Stjepan Radivoj Gost. because he was afraid that this slender will collapse together Mostar developed into a strategic and commercial hub and with its support.

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Historical evidence has shown that the two towers were in method, with a special mixture of 82.50 % cement, 15 % existence in 1452 (some writers describe them as much as mixture of stone flour composed of limestone tenelija and 84 years before the bridge). Research indicate that the towers 2.50 % bentonite. The volume of this mixture required for the where from the era of the medieval Bosnian State and later repairs totalled 7 cubic metres, which gives us a good indica- modified for the needs of the Ottoman Turks. tion that the arch (vault) was severely damaged, and cracks The first renovations of the towers were made during the were also appearing in the stone and mortar joints. At this reign of Mehmed II el-Fatih who also carried out renova- time, damaged stones on the vault as well as on the façade tions on the old wooden bridge existing at that time. During were replaced, and cavities of cement mortar were removed that time the whole town of Mostar was converted into a and replaced with a special mixture that blended in with the fortification. colour of the tenelija stone. First sign on the bridge, dating from 1676, mentions no Shortly before the war, some experiments with silicone repair work on the bridge, but however there is evidence that emulsion were made, but these had no effect apart from leav- repairworkwasinfactcarriedoutatthattime.Thesecond ing ugly traces. sign mentions that repairs on the bridge were completed in 1150 (1736–1737). 2 Description During the Austro-Hungarian occupation of the Mostar area many renovations were made. The sloping ground on The Old Bridge of Mostar (Stari Most) was a stone bridge either side of the bridge was raised by 80 cm. in order to deal of very slender and elegant shapes: its profile and its skyline withthetrafficproblems.Thepathonthebridgewasalsoin- were so thin and so high over the river waters that it was hard creased and re-surfaced. To adapt to the current situation, to believe that such a structure could be worked out of huge the main streets leading to the bridge were raised, and the stone blocks. Built in tenelija stone, it was of a light colour elevation of the entrance doors of the bridge were increased. tone , bright and changeable during daytime, depending on After World War II, reconstruction around the bridge area the colours from the sun. continued and the remains of the Austro-Hungarian inter- The bridge was mainly conceived as a functional structure, ventions were removed. During the Austro-Hungarian period connecting the two banks of the river: it probably did not damaged stones on the bridge were repaired using cement originally have any additional ceremonial and monumental mortar, which was a poor solution as the mortar left stained meaning, and its design was quite influenced by the morphol- dark spots on the bridge. ogy of the site, which is totally matched by its abutment walls. During World War II plans were made to destroy the Old The whole monumental complex, including the adjacent for- Bridge, but fortunately these plans were discarded. However, tification towers, is totally part of the site, castled on the rocks there was some damage, due to mine holes, and even after andwindingthebanks,itisnottheresultofasingledesign repairs of the mortar, water could penetrate the structure work, but of a development over the course of time, that has leading to further damage. The situation was considered followed, and historical events and the need to protect and very serious, and restoration began in 1955. Supports were preserve the bridge, that has marked the town even provided restored on either side of the bridge and damaged walls and its name “Mostar”. empty cavities were fully repaired. The restoration was com- The bridge had few esthetic devices and no ornamental pleted by the end of 1965. The second stage of restoration element; its architectural beauty and value were to be found involved the arch of the Old Bridge, using the injection in the simplicity and in the essentiality of the structure: the shapes of the bridge were not linked to any time, to any style or any fashion, in but the bridge of Mostar has always been admired as symbol. Two cornices, only, with an angled section profile, ran on each elevation and met at the keystone with a refined balance between tangency and jutting out: both of them had a protec- tive function against rain waters, and they marked different structural elements of the monument, being part of it and not pasted. A bigger cornice, on each side, determined the level of the bridge arch springer, prosecuting upstream and down- stream along the abutment walls. The stone surfaces were neither polished nor regular: they were perfectly cut and hand worked, but characterised by frequent ordinary structural inaccuracies which revealed their materiality and the fact that they were made of stone. Of more than one thousand stone blocks, there were no two elements of identical dimensions, and even the arch voussoirs were all different, with remarkable variations, due to the random, naturally-determined, availability of stones in the quarry. Un- doubtedly, most of the architectural beauty of this monument lay in the refined tuning in the globally compact appearance of the structure, caused by the thin joints of the masonry, and Fig. 2: Bridge in the 1980s 1981 project – site plan the unforeseeable and fragmentary close-view appearance

© Czech Technical University Publishing House http://ctn.cvut.cz/ap/ 51 Acta Polytechnica Vol. 46 No. 2/2006 Czech Technical University in Prague due to the small imperfections of stone blocks in the assem- its shape and the configuration of the dead loads above, bling of different shapes and sizes. was subjected to compressive loads. The stone blocks would The one-span bridge had an intrados curvature that was have been perfectly adequate for the purpose, but additional almost circular, with the centre adequate way approximately devices were provided to strengthen the vault: forged iron 296 cm lower than the east springer level; but the circle shape dowels were inserted between adjacent voussoirs, and forged may be not an adequate way to describe the curvature, which iron cramps were placed over the extrados and across the side was modified at the level of the imposts to better match with joints. Thus each connection joint was strengthen either by the straight profile of the abutment wall, and was slightly mortar, or by metal reinforcements. raising at the key stone level. The exact original shape of Over the load-bearing arch there was a masonry rib which the curvature vas desturbed by irregularities that may have together with the spandrel walls stiffened the whole bridge. been caused by settlements and by ordinary construction Between the spandrel walls and the stiffening rib there were imperfections. two lightening voids, which reduced the loads over the arch. Themaindimensionsofthebridgewereasfollows:the Fill was wisely provided only next to the arch springers in span was of 2871 cm on the north side and of 2862 cm on order to stabilise the structure and the vault. The lighten- the south side and the rise of the arch was approximately ing voids were covered by krecnjak stone slabs, and above of 1206 cm. The measurements were clearly often variable, them there were only thin layers of aggregate nad then the and impost level of the bridge on the west bank was approxi- pavement. mately 12–13 cm higher than on the east bank. The appearance and structure of the bridge architecture The load bearing arch had a depth of about 395 cm and were strictly related, in such a way that the inner structure an height of 80 cm, and was composed of 111 rows (a num- could be perceived just by observing of the outer elevations, ber which was probably planned and not randomly obtained), where the cornices marked the most important structural and the rows contained between 2 to 5 voussoirs, (normal- sections of the bridge. ly 3–4). The voussoirs (arch stones) were of variable shapes and sizes, but the average stone block had the following dimensions: 40×80×100 cm. Thearchandelevationsofthebridgeweremadeof tenelija stone (local limestone) and joined by mortar and metal reinforcing cramps and dowels. The use of forged iron devices to strengthen the structure was one of the peculiarities of the monument. They were assembled in various ways, they were applied almost to every element of the bridge. Thebridgearch,atthetop,wasconsiderablyhigher(by about cm 270) than the adjacent street levels. The footpath over the bridge was steep, and tilted in such a way that all the architectural elements, e.g. spandrels, parapets and upper cornices, followed these directions until the top. The spandrel wallsofthebridgeweredividedfromthearchbyastonelower cornice, that followed the arch curvature, and were limited on top by the upper cornice, of straight but tilted profile. The lower cornice stones jutted out from the load bearing Fig. 3: Destroyed bridge – south elevation arch and determined the base from which the spandrel walls started; at the same time the upper cornice was jutted from the spandrel walls. Finally, the parapets were almost aligned with the spandrel walls below them, but leaning slightly, outwards as they approached the top of the bridge, to give the optical effect of a wider footpath. The pavement was made in krecnjak stone (hard, resistant marble-like limestone), and was characterised by transversal rib-steps to prevent slipping; the flooring was assembled on a mortar layer which, most probably, also had a waterproof- ing function together, with the layer below made of “terra rossa” (heated aggregates of red colour due to the presence of bauxite). On the structural side, the bridge was quite interesting, and an analysis of the inner elements shows the level of engineering skills at the time and the clever measures taken to ensure the long life of the structure. Main structural element of the bridge was the load-bearing arch, which was undoubtedly the part of the monument that required the greatest care and resources. The arch, due to Fig. 4: Destroyed bridge – east and west elevation

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3 Destruction 5 Materials used in reconstructing The Old Bridge of Mostar was destroyed in November the arch 1993 by shelling during the war. The shelling was captured l most of the stone elements of the bridge including the on film and some technical data and observations have been vault, cornices, spandrels and parapets, were made in gathered on the basis of this documentation. tenelija stone (Category I), which is a local oolithic lime- An analysis of the remaining portions of the bridge near stone rock of light and warm colour and high porosity, the east abutment, shows thet there was greater surface de- resistant to compressive strengths of about 20 MPa; (from terioration on the north side than on the south side. The the Mukoša quarry); shelling came from the south side, and hit mostly the south l the pavement and the stone slabs over the lightening voids east portion of the bridge over the arch reins: the most exten- were in krecnjak stone, a light-colour hard and resistant siveremainingportionsofthebridgewerelocatedonthat marble-like limestone; side. Shootings was also directed at the north elevation during l other attacks, and for this reason the bridge was protected the metal connectors, cramps and dowels, were in forged by tyres, ( the temporary structures over the footpath were iron as was the fence; intended to protect people from gunfire). Shelling was di- l lead was used for the connectors and for assembling the rected at a single spot in order to destroy the structure with fence; the minimum numbers of shells. The structure was divided l mortars of different types and compositions were found into two main parts: a small part below the arch reins on the all over the structure. Terra rossa – red colour aggregates east bank, and a big part which was totally blown into the with a remarkable content of bauxite were settled in a layer river. beneath the pavement.

4 Reconstruction of the Old Bridge 6 Dismantling the remenants bridge The initiative to rebuild the Old Bridge began soon after One of the first tasks of the contractor for the actual re- the war. As the bridge was one of the masterpieces of Ottoman construction of the bridge was to prepare the Site around the architecture, the Turkish government offered to cover all Old Bridge. The erection of the scaffolding for the side walls the expenses for the reconstruction. However, what the peo- began on 12. 07. 2002. This scaffolding was necessary for the ple of Mostar needed most was to reconstruct the broken repair works on the abutment walls in order to provide a solid bridges between themselves. The city government decided to base for the arch. use this reconstruction to heal the war wounds, and to use After determining the best possible position for placing international experts to promote the importance and value of the main crane, it was necessary to investigate whether the soil this project. beneath it could bear the weight. After the necessary drillings, A Project Coordination Unit (PCU) was formed by the crane foundations were built, and few days later the crane World Bank, UNESCO and City of Mostar, as a specialized came into use. agency responsible for performing professional and other The first actual task on site was to remove the existing tasks to coordinate the project to the build the Old Bridge parts of the arch. The pavement on the right side of the and other structures within the Pilot Cultural Heritage Pro- bridge began to be dismantled on 31.10.2002. Each pave- jects for Bosnia and Herzegovina. ment stone had previously been marked, documented and

pavement / flooring

stone cover slab

lightening void

stiffening rib

arch row

abutment

arch / vault

abutment cornice

Fig. 5: Plan view after destruction Fig. 6: Structural elements of the bridge

It was decided that the entire structure, up to the final wooden bedding,wouldbemadeofsteel,toensuremaximumpreci- sion in imitating the former arch. This was essential, because the tolerance was less than ±1 cm from the recorded coordi- nates of the bridge before destruction. To facilitate movement of people and materials while the centering was being erected, Neretva was temporarily spanned by a pontoon. After the concrete pillars had been produced, preparatory works began on lifting the transversal grids. Between the pillars on both sides, auxiliary steel structures were placed for lifting the grids.

Fig. 7: Dismantling the remnants of the bridge on the south east side measured. Due to its irregular surface and its particularly visible position, the pavement was one of the most delicate dismantling and rebuilding tasks. It was a very delicate job, because it was important to return the largest possible number of stones to their original place. The workers, using hand tools, separated each stone individually from the mortar, silicon and soil.

Fig. 9: South elevation of centring

Transversal girders were mounted on the main steel truss structure (HEB 40), and feet were welded on to it for mount- ing the heavy steel scaffold f 159/150 mm. After the heavy scaffold, which followed the arch, had been mounted it was consolidated with a light scaffold f 48 mm. Height regulators were mounted on the heavy scaffold, and beams of dimensions 24/20 cm, so called covers, on which six wooden semi-circular planks were placed. The final phase was to prepare the floor for the bridge stones.

Fig. 8: Pavement stones in their new location

On 19th November 2002, the remnants of the bridge on the left bank of the Neretva began to be dismantled. This task was very delicate because the console of the bridge that remained after demolition was in very bad condition. Struc- tural fractures were visible. There was no solid support under this part of the bridge for auxiliary structure, which made the job even more difficult. Each dismantled stone was marked and taken away.

7 The centering Fig. 10: Portal crane One of the most important elements in the reconstruction of the Old Bridge was the centering. Because of the unpre- While the centering was being mounted, the portal crane dictable winter and spring levels of the Neretva, the main structure was also being set up. The portal crane was needed steel grids were place above the point of highest possible flow. to ensure that the arch was assembled with precision.

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8 Assembling of the voussoirs 8.1 Dowels in the arch stones The arch voussoirs are the elements of the load bearing Each arch stone was linked to one or two voussoirs in the arch, and are therefore the most important portion of the preceding row by one or more metal dowel. Dowels were bridge. Many reinforcing devices were used to ensure efficient applied to the stone blocks in specially carved slots (holes), joint connections. To better understand how voussoirs were and melted lead was poured into the slots to connect the assembled, it should be taken into account: metal and stone elements. l the load bearing arch was composed of 111 rows of voussoirs 395 cm in thickness; l each row contained 2–5 voussoirs (normally 3–4); l rows of voussoirs were mounted from both sides of the bridge; l the joints of voussoirs belonging to one row were shifted in relation to the preceding row, (as in ordinary masonry work); l a mortar layer was used to join adjacent voussoirs, (as in ordinary masonry work); l additional metal elements were mounted across the joints; l three different metal strengthening devices were used for thearchstones:dowels,sidecrampsandextradoscramps; l metal elements, with purpose-built carvings to host them, had quite variable dimensions and sizes in the bridge Fig. 13: Positions of the dowels in a typical voussoir row structure.

Fig. 14: Typical placing of the cramps in the extrados

8.2 Side cramps in the arch stones The adjacent voussoirs of each row were connected with double side cramps crossing the joints; again, the cramps Fig. 11: Assembling the load bearing arch were linked to the stone with melted lead, poured into specially constructed slots. The side cramps were quite regular, of similar shape and size, and there were fewer variations along the load bearing arch. 8.3 Extrados cramps in the arch stones Extrados cramps were located over the extrados of the loadbearingarch:therewerefiverowsofcrampsthatrun- ning in parallel directions following the curved profile. The extrados cramps acted like tying chains, or at least they were conceived with this purpose, being one adjacent to the other. Thesecrampsweredifferentfromalltheothersthathave been found in the bridge stone structure, because they were dimensioned exclusively according to the arch stone measures: where one cramp ended, another started, sharing the same stone-slot to allow continuity of the tying action. 8.4 Assembling the lower cornices The lower cornices are located directly above the extrados Fig. 12: Load bearing arch completed of the load bearing arch, jutting out from the vault profile to

© Czech Technical University Publishing House http://ctn.cvut.cz/ap/ 55 Acta Polytechnica Vol. 46 No. 2/2006 Czech Technical University in Prague protect the structure. Below the lower cornices, being assembled on a curved profile, required special care during the adjusting procedures, and were anchored to the arch with a mortar layer. On top side of the cornices, across the joints, a single row of cramps was placed approximately along the longitudinal axe of the stone elements.

Fig. 17: Scheme of spandrel stone assembly

Fig. 15: Scheme of cornice

Fig. 18: Assembling the spandrel stones

8.6 Assembling the upper cornices The upper cornices were placed very accurately on a mortar layer over the spandrel walls, to ensure that the top was tangent to the lower cornice, and at the same time, that thecornices were jutting out from the spandrel wall profile to Fig. 16: Assembling the cornices protect them from rainwater. The upper cornices were gener- ally bigger than the lower cornices, and perhaps for this rea- 8.5 Spandrel stones assemblage son, had two rows of cramps instead of one. The cornices were provided with slots and channels that were used to host the The spandrel walls were made in ordinary ashlar masonry dowels of the parapets. work with thin mortar joints. On the top side of the spandrels, across the joints, a single row of cramps was placed approximately along the longitudinal axis of the stone elements, how- 8.7 Assembling the parapets ever the variability of this parameter was quite high, being re- The parapets were assembled over the upper cornices lated to the thickness of the ashlar. The stone block rows were with a mortar layer and with dowels, that previously had to be sometimes assembled with a slight gradient, perhaps due to mountedontheloweredgeoftheparapet.Theywerein- ordinary construction imperfections or in order to optimise of serted in the slots of the upper cornices. The whole was linked the connection with the load bearing arch. by pouring melted lead in the specially carved channels over

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Fig. 22: Scheme of the parapets assembly

thecornices.Onthetopedgeoftheparapetsomeother cramps were placed across the side joints (also linked with mortar). It seems that the parapets towards the top of the bridge were intentionally assembled slanting slightly outwards, perhaps in order to create an optical effect that could give the impression of a wider footpath. 8.8 Assembling of the fence The fences had most probably been assembled in a subse- Fig. 19: Scheme of the upper cornice assembly quent period to protect people from falling. The fences were made of forged iron.

Fig. 20: Assembling the upper cornice

Fig. 23: Lead melting can

Fig. 21: Scheme of the metal elements in the parapets and the fence Fig. 24: Pouring the led

th 9 Lead pouring The archaeological explorations were finished on 6 March 2003, when the excavations reached the rock which is One of the most important materials used in construct- also the last context established under ordinal number 5. Alti- ing the bridge was lead. The bridge itself was an organic tude K-5 on the left bank was 46.90, while on the right bank it structure, capable of resisting various kinds of pressures: was 46.50 in the southern part of the quadrant and 46.30 in strong wind, the river, earthquakes, etc. This kind of flexibil- the northern part. ity was achieved with the use of metal cramps and dowels, The archaeological findings were very interesting on both fixed within the stone cavities with lead. Lead was melted at sides. Several masonry structures were found, some of which 380–390 °C, in a special heater, and poured into the hole us- can be assigned to previous bridges. Most probably two ingasmallcan. bridges had existed at the same spot before the Old Bridge. One of the most important and most delicate procedures These masonry structures were on both sides of the Neretva while building the bridge was pouring the lead into the and they developed at the same time (the objects discovered voussoir stones. Lead was poured through channels into the on one side were also found on the other side). “hidden” (already inserted) dowels between the rows of the In the springer the excavation reached the sterile rocky arch. The channels had to be dry and clean, and the lead had mass layer. A layer of earth more than 8m in height which to be poured at the optimum rate. This dangerous procedure additionally burdened the spandrel walls was excavated, so was performed with 100% success. the walls will no longer be burdened. These excavations enable high quality restoration of the foundation walls on both sides.

11 Recorded findings The oldest finds date from the 15th century. The archaeological finds were recorded in detail in all layers and documentation was made. This dealt with all recorded archaeological findings, contexts, photos and the plan view with cross-sections, and it will be published in a scientific journal. The finds were sent to Croatia for restoration: the metal items to Museum of Croatian Archaeological Monuments in Split, and the wood items to the Croatian Restoration Insti- tute. After treatment they will be returned and placed in the future archaeological museum. Fig. 25: Presumed appearance of the suspension bridge, by Prof. About 200 pieces of pottery were found on the site – dat- Gojkovic ing form the 15th to 20th century, about twenty stone and one metal cannon balls, about 250 metal wedges from the structure of wooden bridge, one copper coin and several mediaeval tools. These studies, which will be continued, have integrated and modified the image of the site development. They have also changed the image of all recent preliminary reports. These finds have changed the flow of construction, and have provided more elements for precise dating of specific walls.

Fig. 26: Traces of the beams for the suspension bridge

10 Archaeological report After a sonar investigation of the towers, which was carried out at the end of 2001 and the beginning of 2002, archaeological work on both sides of the arch restarted in October 2002. Fig. 27: Huge iron nail from the suspension bridge

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References [1] Celic, Dz., Mujezinovic, M.: Stari mostovi u Bosni I Hercegovini. Sarajevo: Sarajevo – Publishing, 1998, p. 228–246. [2] Mutevelic, I.: Mostar. Mostar: Prva knjizevna komuna, 1982. [3] Romeo,M.etal.:Architectural Report, Phase B. Florence: General Engineering, 2000. [4] Krsmanovic, D., Dolarovic, H., Langof, Z.: “Sanation andConservationoftheOldBridgeatMostar.”In:Nase starine XI. Sarajevo: Zavod za zastitu spomenika kulture S. R. Bosne I Hercegovine, 1967. p. 5–23.

Ing. arch. Maja Popovac phone: +387 36 566 112 Fig. 28: Wooden pipe covered in terra rossa mobile: +387 61 204 809 e-mail: [email protected] K. M. V. Humskog 33 12 Conclusion 88000 Mostar, Bosnia and Herzegovina Reconstruction of the Old Bridge of Mostar was a pioneer project in the field of reconstruction. While performing this task, we faced many problems and difficulties on site, and this gave us even more reason to respect the ancient builders. We can only imagine how hard it was to build a masterpiece like this, without any present-day equipment or technology. This reconstruction produced many outcomes: experts and the scientific world gained new insights into construction methods, materials and structures; the word and mankind regained a rare and beautiful monument for the UNESCO list; and last, but certainly not least, the people of Mostar proudly have regained their symbol of peace, multiethnic and multire- ligious coexistence.

Fig. 29: New Old Bridge today